Existing literature has examined factors underlying the formation of goods trade agreements (GTA) and bilateral investment treaties but not the determinants of services trade agreement (STA) membership. This paper bridges the gap by studying the economic and political determinants of STA membership. Its main contribution lies in providing an economic explanation of unilateral services regulatory provisions, embodied in the World Bank’s Services Trade Restrictiveness Index (Borchert et al. in World Bank Econ Rev 28:162–188, 2014), and their interaction with services preferentialism. The authors find that unilateral services provisions are closely associated with economic determinants. They also find that countries’ participation in STAs is correlated with the similarity of their unilateral services trade restrictiveness, a finding not observed for “goods-only” trade agreements. While geographical and cultural determinants are found to be broadly similar for GTAs and STAs, association with economic size of partners, factor endowments and services cost shares in GDP comes through more strongly for goods-only agreements.